High voltage relays require physical isolation for safe operation. Underwriters Laboratories has recently increased its spacing and testing requirements for 347VAC systems. Underwriters Laboratories requires that a certain physical spacing exist between high voltage and any part that could come into contact with a person e.g., an installer or user. Typically, Underwriters Laboratories considers lower voltage circuits, typically 42.5VDC or less and 30VAC or less, to be contactable by a person. Common commercial relays violate the high voltage spacing requirement because the distance from the low voltage coil contacts to the high voltage relay contacts is generally less than the required minimum spacing.
Special relays are available that are Underwriters Laboratories listed for 347VAC operation, but those relays are much larger to meet the spacing requirements, cost three to four times as much as a common commercial relay, and are often a latching-type relay that may not be desired.
In a relay control circuit a low voltage coil, e.g., 6-24V DC or AC, is driven from a low voltage control circuit to establish a current in the coil, thereby establishing a magnetic field that pulls a relay contact armature connected to one high voltage contact toward another open high voltage contact, thereby causing the relay contacts to close and establish a closed high voltage circuit. This allows a high voltage supply on one relay contact to be connected to a load connected to the other contact for the purpose of controlling power to the load. 24VDC is a very common operating voltage in lighting controls, so it is common to find the relay coil being driven from a 24V supply. Most commercial relays are Underwriters Laboratories listed or rated for common U.S. operating voltages, e.g., 120VAC or 277VAC, because the physical spacing between the low voltage coil and the high voltage relay contacts meets Underwriters Laboratories spacing requirements, but those spacings are not suitable for control of 347VAC.